System and method for controlling rotorcraft

ABSTRACT

In an embodiment, a rotorcraft includes: a flight control computer configured to: receive a first sensor signal from a first aircraft sensor of the rotorcraft; receive a second sensor signal from a second aircraft sensor of the rotorcraft, the second aircraft sensor being different from the first aircraft sensor; combine the first sensor signal and the second sensor signal with a complementary filter to determine an estimated vertical speed of the rotorcraft; adjust flight control devices of the rotorcraft according to the estimated vertical speed of the rotorcraft, thereby changing flight characteristics of the rotorcraft; and reset the complementary filter in response to detecting the rotorcraft is grounded.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation-in-part of U.S. patent applicationSer. No. 15/996,132, filed on Jun. 1, 2018, entitled “System and Methodfor Controlling Rotorcraft,” which is incorporated herein by referencein its entirety.

This application also claims the benefit of and priority to U.S.Provisional Patent Application Ser. No. 62/787,602, filed on Jan. 2,2019, entitled “System and Method for Controlling a Rotorcraft,” whichis incorporated herein by reference in its entirety.

BACKGROUND

A rotorcraft may include one or more rotor systems including one or moremain rotor systems. A main rotor system generates aerodynamic lift tosupport the weight of the rotorcraft in flight and thrust to move therotorcraft in forward flight. Another example of a rotorcraft rotorsystem is a tail rotor system. A tail rotor system may generate thrustin the same direction as the main rotor system's rotation to counter thetorque effect created by the main rotor system. For smooth and efficientflight in a rotorcraft, a pilot balances the engine power, main rotorcollective thrust, main rotor cyclic thrust and the tail rotor thrust,and a control system may assist the pilot in stabilizing the rotorcraftand reducing pilot workload.

SUMMARY

In an embodiment, a method includes: receiving a first sensor signalfrom a first aircraft sensor of a rotorcraft, the first sensor signalincluding raw airspeed values of the rotorcraft; receiving a secondsensor signal from a second aircraft sensor of the rotorcraft, thesecond aircraft sensor being different from the first aircraft sensor;measuring a first speed of the rotorcraft, the first speed beinggroundspeed of the rotorcraft or raw airspeed of the rotorcraft;determining airspeed values of the rotorcraft, the determiningincluding: using the raw airspeed values in the first sensor signal asthe determined airspeed values in response to the first speed of therotorcraft being less than or equal to a predetermined value; andestimating the determined airspeed values in response to the first speedof the rotorcraft being greater than the predetermined value, theestimating including: combining the first sensor signal and the secondsensor signal with a complementary filter to determine estimatedairspeed values; and using the estimated airspeed values as thedetermined airspeed values; and adjusting flight control devices of therotorcraft according to the determined airspeed values.

In some embodiments of the method, combining the first sensor signal andthe second sensor signal includes: integrating the second sensor signalwith respect to time to obtain a first filtered signal; and adding thefirst filtered signal and the first sensor signal to obtain a secondfiltered signal, the second filtered signal including the estimatedairspeed values of the rotorcraft. In some embodiments of the method,the first aircraft sensor is a pitot tube. In some embodiments of themethod, the second aircraft sensor is an accelerometer. In someembodiments of the method, the predetermined value is a first thresholdwhen the rotorcraft is accelerating, and the predetermined value is asecond threshold when the rotorcraft is decelerating, the secondthreshold being less than the first threshold. In some embodiments ofthe method, the first threshold is 40 knots, and the second threshold is25 knots.

In an embodiment, a rotorcraft includes: a pitot tube; an accelerometer;a plurality of flight control devices; and a flight control computercoupled to the pitot tube, the accelerometer, and the flight controldevices, the flight control computer being configured to: receive rawairspeed values of the rotorcraft from the pitot tube; receive rawforward acceleration values of the rotorcraft from the accelerometer;measure a first speed of the rotorcraft, the first speed beinggroundspeed of the rotorcraft or raw airspeed of the rotorcraft;determine airspeed values of the rotorcraft by: using the raw airspeedvalues as the determined airspeed values in response to the first speedof the rotorcraft being less than or equal to a predetermined value; andestimating the determined airspeed values by combining the raw airspeedvalues and the raw forward acceleration values in response to the firstspeed of the rotorcraft being greater than the predetermined value; andadjusting flight control devices of the rotorcraft according to thedetermined airspeed values.

In some embodiments of the rotorcraft, the flight control computer isconfigured to estimate the determined airspeed values by: combining theraw airspeed values and the raw forward acceleration values with acomplementary filter to determine estimated airspeed values; and usingthe estimated airspeed values as the determined airspeed values. In someembodiments of the rotorcraft, the flight control computer is configuredto combine the raw airspeed values and the raw forward accelerationvalues by: integrating the raw forward acceleration values with respectto time to obtain integrated forward acceleration values; and adding theintegrated forward acceleration values and the raw airspeed values toobtain the estimated airspeed values. In some embodiments of therotorcraft, the predetermined value is a first threshold when the firstspeed of the rotorcraft is less than or equal to the predeterminedvalue, and the predetermined value is a second threshold when the firstspeed of the rotorcraft is greater than the predetermined value, thesecond threshold being less than the first threshold.

In an embodiment, a method includes: receiving a first sensor signalfrom a first aircraft sensor of a rotorcraft; receiving a second sensorsignal from a second aircraft sensor of the rotorcraft, the secondaircraft sensor being different from the first aircraft sensor;combining the first sensor signal and the second sensor signal with acomplementary filter to determine an estimated speed of the rotorcraft;adjusting flight control devices of the rotorcraft according to theestimated speed of the rotorcraft; measuring a first speed of therotorcraft, the first speed being groundspeed of the rotorcraft or rawairspeed of the rotorcraft; and resetting the complementary filter inresponse to the first speed of the rotorcraft being less than apredetermined threshold.

In some embodiments of the method, the estimated speed of the rotorcraftis an estimate of vertical speed of the rotorcraft. In some embodimentsof the method, the estimated speed of the rotorcraft is an estimate ofairspeed of the rotorcraft. In some embodiments of the method, thecombining the first sensor signal and the second sensor signal with thecomplementary filter includes: filtering the first sensor signal with afirst filter to obtain a first filtered signal; filtering the secondsensor signal with a second filter to obtain a second filtered signal,the second filter and the first filter complementing one another; andadding the first filtered signal and the second filtered signal todetermine the estimated speed of the rotorcraft. In some embodiments ofthe method, the combining the first sensor signal and the second sensorsignal with the complementary filter includes: subtracting the firstsensor signal from the second sensor signal to obtain a noise differencesignal; filtering the noise difference signal to obtain a filtereddifference signal; and adding the first sensor signal and the filtereddifference signal to determine the estimated speed of the rotorcraft. Insome embodiments of the method, the first aircraft sensor is anaccelerometer. In some embodiments of the method, the receiving thefirst sensor signal includes: receiving an acceleration signal from theaccelerometer; and integrating the acceleration signal to obtain thefirst sensor signal. In some embodiments of the method, the secondaircraft sensor is a pitot tube. In some embodiments of the method, thereceiving the second sensor signal includes: receiving a velocity signalfrom the pitot tube; and using the velocity signal as the second sensorsignal. In some embodiments of the method, the adjusting the flightcontrol devices of the rotorcraft includes: decoupling pilot flightcontrols of the rotorcraft from the flight control devices according tothe estimated speed. In some embodiments of the method, the first speedis the groundspeed of the rotorcraft. In some embodiments of the method,the first speed is the raw airspeed of the rotorcraft.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a rotorcraft, according to some embodiments;

FIG. 2A is a block diagram of a fly-by-wire flight control system,according to some embodiments;

FIG. 2B illustrates the inside of a cockpit, according to someembodiments;

FIG. 3 is a block diagram of a three-loop flight control system,according to some embodiments;

FIG. 4 is a block diagram of a first system for vertical speedestimation, according to some embodiments;

FIG. 5 is a block diagram of a second system for vertical speedestimation, according to some embodiments;

FIG. 6 is a block diagram of a method for vertical speed estimation,according to some embodiments;

FIG. 7 is a block diagram of a filter, according to some embodiments;

FIG. 8 is a block diagram of a method for airspeed estimation, accordingto some embodiments; and

FIG. 9 is a block diagram of a filter, according to some embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Illustrative embodiments of the system and method of the presentdisclosure are described below. In the interest of clarity, all featuresof an actual implementation may not be described in this specification.It will of course be appreciated that in the development of any suchactual embodiment, numerous implementation-specific decisions may bemade to achieve the developer's specific goals, such as compliance withsystem-related and business-related constraints, which will vary fromone implementation to another. Moreover, it should be appreciated thatsuch a development effort might be complex and time-consuming but wouldnevertheless be a routine undertaking for those of ordinary skill in theart having the benefit of this disclosure.

Reference may be made herein to the spatial relationships betweenvarious components and to the spatial orientation of various aspects ofcomponents as the devices are depicted in the attached drawings.However, as will be recognized by those skilled in the art after acomplete reading of the present disclosure, the devices, members,apparatuses, etc. described herein may be positioned in any desiredorientation. Thus, the use of terms such as “above,” “below,” “upper,”“lower,” or other like terms to describe a spatial relationship betweenvarious components or to describe the spatial orientation of aspects ofsuch components should be understood to describe a relative relationshipbetween the components or a spatial orientation of aspects of suchcomponents, respectively, as the device described herein may be orientedin any desired direction.

The increasing use of rotorcraft, in particular, for commercial andindustrial applications, has led to the development of larger morecomplex rotorcraft. However, as rotorcraft become larger and morecomplex, the differences between flying rotorcraft and fixed wingaircraft has become more pronounced. Since rotorcraft use one or moremain rotors to simultaneously provide lift, control attitude, controlaltitude, and provide lateral or positional movement, different flightparameters and controls are tightly coupled to each other, as theaerodynamic characteristics of the main rotors affect each control andmovement axis. For example, the flight characteristics of a rotorcraftat cruising speed or high speed may be significantly different than theflight characteristics at hover or at relatively low speeds.Additionally, different flight control inputs for different axes on themain rotor, such as cyclic inputs or collective inputs, affect otherflight controls or flight characteristics of the rotorcraft. Forexample, pitching the nose of a rotorcraft forward to increase forwardspeed will generally cause the rotorcraft to lose altitude. In such asituation, the collective may be increased to maintain level flight, butthe increase in collective requires increased power at the main rotorwhich, in turn, requires additional anti-torque force from the tailrotor. This is in contrast to fixed wing systems where the controlinputs are less closely tied to each other and flight characteristics indifferent speed regimes are more closely related to each other.

Recently, fly-by-wire (FBW) systems have been introduced in rotorcraftto assist pilots in stably flying the rotorcraft and to reduce workloadon the pilots. The FBW system may provide different controlcharacteristics or responses for cyclic, pedal or collective controlinput in the different flight regimes, and may provide stabilityassistance or enhancement by decoupling physical flight characteristicsso that a pilot is relieved from needing to compensate for some flightcommands issued to the rotorcraft. FBW systems may be implemented in oneor more flight control computers (FCCs) disposed between the pilotcontrols and flight control systems, providing corrections to flightcontrols that assist in operating the rotorcraft more efficiently orthat put the rotorcraft into a stable flight mode while still allowingthe pilot to override the FBW control inputs. The FBW systems in arotorcraft may, for example, automatically adjust power output by theengine to match a collective control input, apply collective or powercorrection during a cyclic control input, provide automation of one ormore flight control procedures, provide for default or suggested controlpositioning, or the like.

FBW systems for rotorcraft must provide stable flight characteristicsfor FBW system controlled flight parameters while permitting the pilotto override or adjust any suggested flight parameters suggested by theFBW system. Additionally, in providing enhanced control and automatedfunctionality for rotorcraft flight, the FBW system must maintain anintuitive and easy to use flight control system for the pilot. Thus, theFBW system adjusts the pilot flight controls so that the controls are ina position associated with the relevant flight parameter. For example,the FBW system may adjust the collective stick to provide suggested orFBW system controlled flight parameters, and which reflect a collectiveor power setting. Thus, when the pilot releases the collective stick andthe FBW system provides collective control commands, the collectivestick is positioned intuitively in relation to the actual power orcollective setting so that, when the pilot grasps the collective stickto retake control, the control stick is positioned where the pilotexpects the stick to be positioned for the actual collective setting ofthe main rotor. Similarly, the FBW system use the cyclic stick to, forexample, adjust for turbulence, drift or other disturbance to the flightpath, and may move the cyclic stick as the FBW system compensates thecyclic control. Thus, when the pilot grasps the cyclic stick to takecontrol of flight from the FBW system, the cyclic stick is positioned toreflect the actual cyclic settings.

FIG. 1 illustrates a rotorcraft 101, according to some embodiments. Therotorcraft 101 has a main rotor system 103, which includes a pluralityof main rotor blades 105. The pitch of each main rotor blade 105 may becontrolled by a swashplate 107 in order to selectively control theattitude, altitude and movement of the rotorcraft 101. The swashplate107 may be used to collectively and/or cyclically change the pitch ofthe main rotor blades 105. The rotorcraft 101 also has an anti-torquesystem, which may include a tail rotor 109, no-tail-rotor (NOTAR), ordual main rotor system. In rotorcraft with a tail rotor 109, the pitchof each tail rotor blade 111 is collectively changed in order to varythrust of the anti-torque system, providing directional control of therotorcraft 101. The pitch of the tail rotor blades 111 is changed by oneor more tail rotor actuators. In some embodiments, the FBW system sendselectrical signals to the tail rotor actuators or main rotor actuatorsto control flight of the rotorcraft.

Power is supplied to the main rotor system 103 and the anti-torquesystem by engines 115. There may be one or more engines 115, which maybe controlled according to signals from the FBW system. The output ofthe engine 115 is provided to a driveshaft 117, which is mechanicallyand operatively coupled to the main rotor system 103 and the anti-torquesystem through a main rotor transmission 119 and a tail rotortransmission, respectively.

The rotorcraft 101 further includes a fuselage 125 and tail section 123.The tail section 123 may have other flight control devices such ashorizontal or vertical stabilizers, rudder, elevators, or other controlor stabilizing surfaces that are used to control or stabilize flight ofthe rotorcraft 101. The fuselage 125 includes a cockpit 127, whichincludes displays, controls, and instruments. It should be appreciatedthat even though rotorcraft 101 is depicted as having certainillustrated features, the rotorcraft 101 may have a variety ofimplementation-specific configurations. For instance, in someembodiments, cockpit 127 is configured to accommodate a pilot or a pilotand co-pilot, as illustrated. It is also contemplated, however, thatrotorcraft 101 may be operated remotely, in which case cockpit 127 couldbe configured as a fully functioning cockpit to accommodate a pilot (andpossibly a co-pilot as well) to provide for greater flexibility of use,or could be configured with a cockpit having limited functionality(e.g., a cockpit with accommodations for only one person who wouldfunction as the pilot operating perhaps with a remote co-pilot or whowould function as a co-pilot or back-up pilot with the primary pilotingfunctions being performed remotely). In yet other contemplatedembodiments, rotorcraft 101 could be configured as an unmanned vehicle,in which case cockpit 127 could be eliminated entirely in order to savespace and cost.

FIG. 2A is a block diagram of a fly-by-wire flight control system 201for the rotorcraft 101, according to some embodiments. FIG. 2Billustrates the inside of the cockpit 127, according to someembodiments, and is described in conjunction with FIG. 2A. A pilot maymanipulate one or more pilot flight controls in order to control flightof the rotorcraft. The pilot flight controls may include manual controlssuch as a cyclic stick 231 in a cyclic control assembly 217, acollective stick 233 in a collective control assembly 219, and pedals239 in a pedal control assembly 221. Inputs provided by the pilot to thepilot flight controls may be transmitted mechanically and/orelectronically (e.g., via the FBW flight control system) to flightcontrol devices by the flight control system 201. Flight control devicesmay represent devices operable to change the flight characteristics ofthe rotorcraft 101. Flight control devices on the rotorcraft may includemechanical and/or electrical systems operable to change the positions orangle of attack of the main rotor blades 105 and the tail rotor blades111 or to change the power output of the engines 115, as examples.Flight control devices include systems such as the swashplate 107, tailrotor actuator 113, and systems operable to control the engines 115. Theflight control system 201 may adjust the flight control devicesindependently of the flight crew in order to stabilize the rotorcraft,reduce workload of the flight crew, and the like. The flight controlsystem 201 includes engine control computers (ECCUs) 203, flight controlcomputers (FCCs) 205, and aircraft sensors 207, which collectivelyadjust the flight control devices.

The flight control system 201 has one or more FCCs 205. In someembodiments, multiple FCCs 205 are provided for redundancy. One or moremodules within the FCCs 205 may be partially or wholly embodied assoftware and/or hardware for performing any functionality describedherein. In embodiments where the flight control system 201 is a FBWflight control system, the FCCs 205 may analyze pilot inputs anddispatch corresponding commands to the ECCUs 203, the tail rotoractuator 113, and/or actuators for the swashplate 107. Further, the FCCs205 are configured and receive input commands from the pilot controlsthrough sensors associated with each of the pilot flight controls. Theinput commands are received by measuring the positions of the pilotcontrols. The FCCs 205 also control tactile cueing commands to the pilotcontrols or display information in instruments on, for example, aninstrument panel 241.

The ECCUs 203 control the engines 115. For example, the ECCUs 203 mayvary the output power of the engines 115 to control the rotational speedof the main rotor blades or the tail rotor blades. The ECCUs 203 maycontrol the output power of the engines 115 according to commands fromthe FCCs 205, or may do so based on feedback such as measuredrevolutions per minute (RPM) of the main rotor blades.

The aircraft sensors 207 are in communication with the FCCs 205. Theaircraft sensors 207 may include sensors for measuring a variety ofrotorcraft systems, flight parameters, environmental conditions and thelike. For example, the aircraft sensors 207 may include sensors formeasuring airspeed, altitude, attitude, position, orientation,temperature, airspeed, vertical speed, and the like. Other aircraftsensors 207 could include sensors relying upon data or signalsoriginating external to the rotorcraft, such as a global positioningsystem (GPS) sensor, a VHF Omnidirectional Range sensor, InstrumentLanding System (ILS), and the like.

The cyclic control assembly 217 is connected to a cyclic trim assembly229 having one or more cyclic position sensors 211, one or more cyclicdetent sensors 235, and one or more cyclic actuators or cyclic trimmotors 209. The cyclic position sensors 211 measure the position of thecyclic stick 231. In some embodiments, the cyclic stick 231 is a singlecontrol stick that moves along two axes and permits a pilot to controlpitch, which is the vertical angle of the nose of the rotorcraft androll, which is the side-to-side angle of the rotorcraft. In someembodiments, the cyclic control assembly 217 has separate cyclicposition sensors 211 that measuring roll and pitch separately. Thecyclic position sensors 211 for detecting roll and pitch generate rolland pitch signals, respectively, (sometimes referred to as cycliclongitude and cyclic latitude signals, respectively) which are sent tothe FCCs 205, which controls the swashplate 107, engines 115, tail rotor109 or related flight control devices.

The cyclic trim motors 209 are connected to the FCCs 205, and receivesignals from the FCCs 205 to move the cyclic stick 231. In someembodiments, the FCCs 205 determine a suggested cyclic stick positionfor the cyclic stick 231 according to one or more of the collectivestick position, the pedal position, the speed, altitude and attitude ofthe rotorcraft, the engine revolutions per minute (RPM), enginetemperature, main rotor RPM, engine torque or other rotorcraft systemconditions or flight conditions. The suggested cyclic stick position isa position determined by the FCCs 205 to give a desired cyclic action.In some embodiments, the FCCs 205 send a suggested cyclic stick positionsignal indicating the suggested cyclic stick position to the cyclic trimmotors 209. While the FCCs 205 may command the cyclic trim motors 209 tomove the cyclic stick 231 to a particular position (which would in turndrive actuators associated with swashplate 107 accordingly), the cyclicposition sensors 211 detect the actual position of the cyclic stick 231that is set by the cyclic trim motors 209 or input by the pilot,allowing the pilot to override the suggested cyclic stick position. Thecyclic trim motor 209 is connected to the cyclic stick 231 so that thepilot may move the cyclic stick 231 while the trim motor is driving thecyclic stick 231 to override the suggested cyclic stick position. Thus,in some embodiments, the FCCs 205 receive a signal from the cyclicposition sensors 211 indicating the actual cyclic stick position, and donot rely on the suggested cyclic stick position to command theswashplate 107.

Similar to the cyclic control assembly 217, the collective controlassembly 219 is connected to a collective trim assembly 225 having oneor more collective position sensors 215, one or more collective detentsensors 237, and one or more collective actuators or collective trimmotors 213. The collective position sensors 215 measure the position ofa collective stick 233 in the collective control assembly 219. In someembodiments, the collective stick 233 is a single control stick thatmoves along a single axis or with a lever type action. A collectiveposition sensor 215 detects the position of the collective stick 233 andsends a collective position signal to the FCCs 205, which controlsengines 115, swashplate actuators, or related flight control devicesaccording to the collective position signal to control the verticalmovement of the rotorcraft. In some embodiments, the FCCs 205 may send apower command signal to the ECCUs 203 and a collective command signal tothe main rotor or swashplate actuators so that the angle of attack ofthe main blades is raised or lowered collectively, and the engine poweris set to provide the needed power to keep the main rotor RPMsubstantially constant.

The collective trim motor 213 is connected to the FCCs 205, and receivessignals from the FCCs 205 to move the collective stick 233. Similar tothe determination of the suggested cyclic stick position, in someembodiments, the FCCs 205 determine a suggested collective stickposition for the collective stick 233 according to one or more of thecyclic stick position, the pedal position, the speed, altitude andattitude of the rotorcraft, the engine RPM, engine temperature, mainrotor RPM, engine torque or other rotorcraft system conditions or flightconditions. The FCCs 205 generate the suggested collective stickposition and send a corresponding suggested collective stick signal tothe collective trim motors 213 to move the collective stick 233 to aparticular position. The collective position sensors 215 detect theactual position of the collective stick 233 that is set by thecollective trim motor 213 or input by the pilot, allowing the pilot tooverride the suggested collective stick position.

The pedal control assembly 221 has one or more pedal sensors 227 thatmeasure the position of pedals or other input elements in the pedalcontrol assembly 221. In some embodiments, the pedal control assembly221 is free of a trim motor or actuator, and may have a mechanicalreturn element that centers the pedals when the pilot releases thepedals. In other embodiments, the pedal control assembly 221 has one ormore trim motors that drive the pedal to a suggested pedal positionaccording to a signal from the FCCs 205. The pedal sensor 227 detectsthe position of the pedals 239 and sends a pedal position signal to theFCCs 205, which controls the tail rotor 109 to cause the rotorcraft toyaw or rotate around a vertical axis.

The cyclic trim motors 209 and collective trim motors 213 may drive thecyclic stick 231 and collective stick 233, respectively, to suggestedpositions. The cyclic trim motors 209 and collective trim motors 213 maydrive the cyclic stick 231 and collective stick 233, respectively, tosuggested positions, but this movement capability may also be used toprovide tactile cueing to a pilot. The cyclic trim motors 209 andcollective trim motors 213 may push the respective stick in a particulardirection when the pilot is moving the stick to indicate a particularcondition. Since the FBW system mechanically disconnects the stick fromone or more flight control devices, a pilot may not feel a hard stop,vibration, or other tactile cue that would be inherent in a stick thatis mechanically connected to a flight control assembly. In someembodiments, the FCCs 205 may cause the cyclic trim motors 209 andcollective trim motors 213 to push against a pilot command so that thepilot feels a resistive force, or may command one or more frictiondevices to provide friction that is felt when the pilot moves the stick.Thus, the FCCs 205 control the feel of a stick by providing pressureand/or friction on the stick.

Additionally, the cyclic control assembly 217, collective controlassembly 219 and/or pedal control assembly 221 may each have one or moredetent sensors that determine whether the pilot is handling a particularcontrol device. For example, the cyclic control assembly 217 may have acyclic detent sensor 235 that determines that the pilot is holding thecyclic stick 231, while the collective control assembly 219 has acollective detent sensor 237 that determines whether the pilot isholding the collective stick 233. The cyclic detent sensor 235 andcollective detent sensor 237 detect motion and/or position of therespective control stick that is caused by pilot input, as opposed tomotion and/or position caused by commands from the FCCs 205, rotorcraftvibration, and the like and provide feedback signals indicative of suchto the FCCs 205. When the FCCs 205 detect that a pilot has control of,or is manipulating, a particular control, the FCCs 205 may determinethat stick to be out-of-detent (OOD). Likewise, the FCCs may determinethat the stick is in-detent (ID) when the signals from the detentsensors indicate to the FCCs 205 that the pilot has released aparticular stick. The FCCs 205 may provide different default control orautomated commands to one or more flight systems based on the detentstatus of a particular stick or pilot control.

FIG. 3 is a block diagram of the flight control system 201, according tosome embodiments. Some operational aspects of the flight control system201 are shown in a highly schematic fashion. In particular, the flightcontrol system 201 is schematically shown as being implemented as aseries of inter-related feedback loops running certain control laws.Although the flight control system 201 is illustrated as being athree-loop flight control system, it should be appreciated that theflight control system 201 could be implemented in a different manner,such as with a different quantity of control loops.

In some embodiments, elements of the flight control system 201 may beimplemented at least partially by the FCCs 205. However, all, some, ornone of the components (301, 303, 305, 307) of flight control system 201could be located external or remote from the rotorcraft 101 andcommunicate to on-board devices through a network connection 309.

The flight control system 201 has a pilot input 311, an outer loop 313,a middle loop 315, an inner loop 317, a decoupler 319, and aircraftequipment 321 (corresponding, e.g., to flight control devices such asswashplate 107, tail rotor transmission 121, etc.; to actuators (notshown) driving the flight control devices; to sensors such as aircraftsensors 207, cyclic position sensors 211, collective position sensors215, cyclic detent sensors 235, collective detent sensors 237, etc.; andthe like).

In the example shown, a three-loop design separates the innerstabilization and rate feedback loops from outer guidance and trackingloops. The control law structure primarily assigns the overallstabilization task and related tasks of reducing pilot workload to innerloop 317. Next, the middle loop 315 (sometimes called the rate loop)provides rate augmentation. Outer loop 313 focuses on guidance andtracking tasks. Since inner loop 317 and middle loop 315 provide most ofthe stabilization, less control effort is required at the outer looplevel. As representatively illustrated, a switch 323 may be provided toturn outer loop flight augmentation on and off, the tasks of outer loop313 are not necessary for flight stabilization.

In some embodiments, the inner loop 317 and middle loop 315 include aset of gains and filters applied to roll/pitch/yaw 3-axis rate gyro andacceleration feedback sensors. Both the inner loop and rate loop maystay active, independent of various outer loop hold modes. Outer loop313 may include cascaded layers of loops, including an attitude loop, aspeed loop, a position loop, a vertical speed loop, an altitude loop,and a heading loop. According to some embodiments, the control lawsrunning in the illustrated the loops allow for decoupling of otherwisecoupled flight characteristics, which in turn may provide for morestable flight characteristics and reduced pilot workload. Furthermore,the outer loop 313 may allow for automated or semi-automated operationof certain high-level tasks or flight patterns, thus further relievingthe pilot workload and allowing the pilot to focus on other mattersincluding observation of the surrounding terrain.

The flight control system 201 may be realized as programming executed bythe FCCs 205. The programming includes instructions implementing aspectsof the flight control system 201. The FCCs 205 may include memories 325,such as non-transitory computer readable storage mediums, that store theprogramming. One or more processors 327 are connected to the memories325, and are operable to execute the programming.

The flight control system 201 relies on accurately determining theposition, velocity (or speed), and acceleration of the rotorcraft 101when controlling flight. In particular, all three loops of the flightcontrol system 201 use the vertical speed (V_(V_SPD)) of the rotorcraft101. The use of inaccurate V_(V_SPD) values may degrade the performanceof the control laws implemented by the flight control system 201, andmay result in partial or total failure of the rotorcraft 101. Accordingto some embodiments, V_(V_SPD) is estimated by performing complementaryfiltering with multiple signals containing values for V_(V_SPD).Estimating the V_(V_SPD) of the rotorcraft 101 by complementaryfiltering with multiple signals may improve the accuracy of V_(V_SPD)values used by the flight control system 201.

FIG. 4 is a block diagram of a first system 401 for V_(V_SPD)estimation, according to some embodiments. The first system 401 has twoinputs: a first measured vertical speed (V_(M1)) signal and a secondmeasured vertical speed (V_(M2)) signal. The signals may be receivedfrom a variety of different sources, and as will be discussed below, maybe indirectly determined by measuring other properties of the rotorcraft101. The signals are noisy signals containing values for V_(V_SPD). Thenoise in V_(M1) and V_(M2) may be located in different regions. Inparticular, V_(M1) includes accurate V_(V_SPD) values in the short-termbut is noisy in the long-term (e.g., has low-frequency noise).Conversely, V_(M2) includes accurate V_(V_SPD) values in the long-termbut is noisy in the short-term (e.g., has high-frequency noise). Thefirst system 401 includes a high-pass filter 403, a low-pass filter 405,and a summer 405, each of which may be implemented as analog circuits orthrough digital signal processing in the FCCs 205, such as with adigital signal processor (DSP).

The high-pass filter 403 is used to filter low-frequency noise fromV_(M1), and the low-pass filter 405 is used to filter high-frequencynoise from V_(M2). Notably, the high-pass filter 403 and low-pass filter405 are complementary filters. For example, if the transfer function ofthe high-pass filter 403 is F(s), then the transfer function of thelow-pass filter 405 is the complement of F(s), e.g., 1−F(s). In otherwords, the high-pass filter 403 and low-pass filter 405 sum to anall-pass filter having a value of 1. The summer 405 adds the filteredsignals from the high-pass filter 403 and low-pass filter 405, therebyproducing an estimated vertical speed (V_(V_EST)) signal. Although theV_(V_EST) signal contains an estimation of V_(V_SPD), it may containmore accurate V_(V_SPD) values than the noisy signals V_(M1) and V_(M2)contain individually.

FIG. 5 is a block diagram of a second system 501 for V_(V_SPD)estimation, according to some embodiments. Like the first system 401,the second system 501 has two inputs (V_(M1) and V_(M2)) and one output(V_(V_EST)). The second system 501 is equivalent to the first system401, except the second system 501 only has a single filter 503 thatoperates on noise in the V_(M1) and V_(M2) signals. A summer 505determines the difference between the input V_(M1) and V_(M2) signals(e.g., subtracts V_(M1) from V_(M2)), and the filter 503 operates onthat difference. A summer 507 then sums the filtered difference with theoriginal V_(M1) signal, thereby producing V_(V_EST) as its output.

It should be appreciated that the first system 401 and second system 501show some examples of complementary filters. Other configuration (suchas that shown below in FIG. 7) may also be used to implementcomplementary filtering.

FIG. 6 is a block diagram of a method 601 for vertical speed estimation,according to some embodiments. The method 601 may be implemented as partof the flight control system 201. In particular, the FCCs 205 mayperform the method 601 when controlling flight of the rotorcraft 101.

In process 603, the V_(M1) signal is received. It should be appreciatedthat the V_(M1) signal may be received through a variety of means orsources. In some embodiments, the V_(M1) signal is directly transduced.For example, the V_(M1) signal may be a sensor signal from an aircraftsensor 207 that directly measures vertical speed and produces a rawvertical speed value, such as with a variometer or other vertical speedindicator (VSI). In other embodiments, another sensor signal istransduced and the V_(M1) signal is derived from the transduced sensorsignal. For example, the V_(M1) signal may be indirectly received bymeasuring vertical inertial acceleration (e.g., a(t)) of the rotorcraft101. Vertical inertial acceleration may be measured with a first one ofthe aircraft sensors 207, such as with an accelerometer. The V_(M1)signal is then obtained by integrating the vertical inertialacceleration with respect to time, as shown in Equation 1. DeterminingV_(V_SPD) of the rotorcraft 101 by integrating the vertical inertialacceleration may be more accurate in the short-term than in thelong-term.V _(M1) =∫a(t)dt  (1)

In process 605, the V_(M2) signal is received. It should be appreciatedthat the V_(M2) signal may be received through a variety of means orsources. In some embodiments, the V_(M2) signal is directly transduced.For example, the V_(M2) signal may be a sensor signal from an aircraftsensor 207 that directly measures vertical speed and produces a rawvertical speed value, such as with a variometer or other VSI. In otherembodiments, another sensor signal is transduced and the V_(M2) signalis derived from the transduced sensor signal. Different aircraft sensor207 may be used to receive the V_(M1) and V_(M2) signals. For example,the V_(M2) signal may be indirectly received by measuring verticalposition (e.g., s(t)) of the rotorcraft 101. Vertical position may bemeasured with a second one of the aircraft sensors 207, such as with anair pressure sensor (e.g., a barometer), a GPS sensor, an ultrasonic orlaser-based height measurement sensor, or the like. The V_(M2) signal isthen obtained by differentiating the vertical position with respect totime, as shown in Equation 2. Determining V_(V_SPD) of the rotorcraft101 by differentiating the vertical position may be more accurate in thelong-term than in the short-term.

$\begin{matrix}{V_{M\; 2} = {\frac{d}{dt}{s(t)}}} & (2)\end{matrix}$

In process 607, the V_(M1) and V_(M2) signals are combined withcomplementary filtering to determine a V_(V_EST) signal, which indicatesthe V_(V_EST) of the rotorcraft 101. The complementary filtering may beperformed with a system such as the first system 401, the second system501, or the like. As noted above, such filtering could be performed inhardware or in software.

In process 609, flight of the rotorcraft 101 is controlled based onV_(V_EST) of the rotorcraft 101. The V_(V_EST) value may be a variableused by one or more loops of the flight control system 201, such as theouter loop 313, middle loop 315, and/or inner loop 317. The flightcontrol devices of the rotorcraft 101 may be adjusted to change theflight characteristics of the rotorcraft 101. For example, theswashplate 107 may be actuated based on V_(V_EST) to maintain aparticular speed or direction, to automate an approach to hover orlanding, or the like. Likewise, the tail rotor actuator 113 may beadjusted based on V_(V_EST). In some embodiments, the value of V_(V_EST)may be used to influence other functionality of the flight controlsystem 201. For example, the decoupler 319 may use V_(V_EST) whendecoupling pilot flight controls, or the middle loop 315 may useV_(V_EST) when stabilizing the rotorcraft 101.

FIG. 7 is a block diagram of a filter 701 that may be used to realizethe first system 401 or second system 501, according to someembodiments. The filter 701 is a complimentary filter implemented withthree integrators 703, 705, and 707. Further, the filter 701 isindicative of embodiments where the V_(M1) and V_(M2) signals arederived from transduced sensor signals. In particular, the V_(M1) signalis derived from a signal from an altimeter (e.g., s(t)), and the V_(M2)signal is derived from a signal from an accelerometer (e.g., a(t)). Thefilter 701 produces two output signals: V_(V_EST) and s(t)_(EST). Thes(t)_(EST) signal is an estimate of the altimeter signal, which is alsofiltered by complimentary filtering.

Resetting the integrators 703, 705, and 707 may ensure the filter 701accurately produces the V_(V_EST) signal. The integrators 703, 705, and707 may be reset in a variety of ways, and may be reset to differentvalues. According to some embodiments, the integrators 703, 705, and 707may be reset when: (1) the FCCs 205 initially start up; (2) the FCCs 204are reset; (3) the rotorcraft 101 is on the ground, as determined byweight-on-wheel sensors of the aircraft sensors 207; (4) an input signalof the filter 701 has failed; (5) any one of the integrators 703, 705,and 707 have reached their upper or lower bounds, which may bepredefined values; and/or (6) the groundspeed or raw airspeed of therotorcraft 101 is less than a predefined threshold.

In some embodiments, the filter 701 is reset in response to detectingthe rotorcraft is grounded. Detecting the rotorcraft is grounded mayinclude receiving a first weight signal from a weight-on-wheel sensor ofthe rotorcraft 101; determining, according to the first weight signal, afirst wheel of the rotorcraft 101 is bearing more than a predeterminedquantity of weight; and determining the rotorcraft 101 is grounded inresponse to the first wheel bearing more than the predetermined quantityof weight.

Upon a reset, the integrator 703 is reset to a value of zero, theintegrator 705 is reset to the current vertical speed value as estimatedby air data systems of the rotorcraft 101, and the integrator 707 isreset to the current altitude as estimated by air data systems of therotorcraft 101. The air data systems of the rotorcraft 101 determine thevertical speed and altitude directly by transducing signals from airsensors of the aircraft sensors 207.

Although embodiments are described in the context of estimating thevertical speed (V_(V_SPD)) of the rotorcraft 101, it should beappreciated that embodiment techniques may be used to estimate speedsalong other directions. For example, the airspeed of the rotorcraft 101may also be estimated by complimentary filtering with multiple noisysignals. All three loops of the flight control system 201 also use theairspeed of the rotorcraft 101. The use of inaccurate airspeed valuesmay degrade the performance of the control laws implemented by theflight control system 201, and may result in partial or total failure ofthe rotorcraft 101. According to some embodiments, the airspeed of therotorcraft 101 may be estimated by performing complementary filteringwith multiple signals containing values for the airspeed, e.g., signalswith high-frequency noise and signals with low-frequency noise.Estimating the airspeed of the rotorcraft 101 by complementary filteringwith multiple signals may improve the accuracy of airspeed values usedby the flight control system 201.

Although estimating the various speeds of the rotorcraft 101 bycombining multiple different signals may generally result in moreaccurate speed values, the estimated speed values may be inaccurate insome conditions. When the inputs to the complementary filter diverge bya large amount, the estimated airspeed of the rotorcraft 101 may beinaccurate. In some embodiments, estimation of the airspeed bycomplementary filtering may only be performed under conditions where theestimated airspeed is likely to be accurate. For example, the inputs tothe complementary filter for estimating the airspeed may diverge by alarge amount when the rotorcraft 101 is operating in hover, in highwinds. In such conditions, the pilot of the rotorcraft 101 may attemptto counteract high winds by quickly changing the heading of therotorcraft 101, e.g., by actuating the pedals 239. Executing such aquick maneuver may cause pitot-type sensors to falsely indicate a rapidchange in longitudinal airspeed without a corresponding change inlongitudinal acceleration. During such a maneuver, the longitudinalairspeed of the rotorcraft 101 estimated by the complimentary filter maybe inaccurate. A similar phenomena may also occur for lateral airspeed.When the estimated airspeed of the rotorcraft 101 is inaccurate,directly measuring the airspeed of the rotorcraft 101 with, e.g., apitot tube, an airspeed indicator (ASI), or the like may provide moreaccurate airspeed values. Many conditions that would result ininaccurate airspeed estimation occur at low groundspeeds or low rawairspeeds. In some embodiments, airspeed estimation by complementaryfiltering may only be performed when the groundspeed of the rotorcraft101 is greater than a predefined threshold. Likewise, in someembodiments, airspeed estimation by complementary filtering may only beperformed when the raw airspeed of the rotorcraft 101 is greater than apredefined threshold.

FIG. 8 is a block diagram of a method 801 for controlling a rotorcraft,according to some embodiments. The method 801 may be implemented as partof the flight control system 201. In particular, the FCCs 205 mayperform the method 801 when controlling flight of the rotorcraft 101.

In process 803, the groundspeed of the rotorcraft 101 is measured. Thegroundspeed may be measured by a GPS, a VHF Omnidirectional Rangesensor, or the like. In process 805, the groundspeed of the rotorcraft101 is compared to a predetermined threshold. In some embodiments, thepredetermined threshold is in the range from 25 knots to 40 knots. Itshould be appreciated that the predetermined threshold may be varied tooptimize response in a given rotorcraft application.

When the groundspeed of the rotorcraft 101 is greater than thepredetermined threshold, in process 807, the airspeed of the rotorcraftis estimated by combining multiple different signals with acomplimentary filter. In some embodiments, the complimentary filtercombines a raw airspeed signal from, e.g., an ASI, and a derivedairspeed signal from, e.g., an accelerometer. As noted above, one of thesignals may have high-frequency noise, and another of the signals mayhave low-frequency noise. In process 809, flight of the rotorcraft 101is controlled based on the estimated airspeed (V_(A_EST)) of therotorcraft 101. One or more loops of the flight control system 201, suchas the outer loop 313, middle loop 315, and/or inner loop 317 rely on adetermined airspeed value for the rotorcraft 101. The flight controldevices of the rotorcraft 101 may be adjusted based on the determinedairspeed value to change the flight characteristics of the rotorcraft101. The V_(A_EST) value is used by the control loops for the determinedairspeed.

When the groundspeed of the rotorcraft 101 is less than thepredetermined threshold, in process 811, the airspeed of the rotorcraftis directly measured by receiving a raw airspeed value from, e.g., apitot tube, an ASI, or the like. Complimentary filtering with multiplesignals is not performed when directly measuring the airspeed of therotorcraft 101. In process 813, flight of the rotorcraft 101 iscontrolled based on the raw airspeed value (V_(A_RAW)) of therotorcraft. One or more loops of the flight control system 201, such asthe outer loop 313, middle loop 315, and/or inner loop 317 rely on adetermined airspeed value for the rotorcraft 101. The flight controldevices of the rotorcraft 101 may be adjusted based on the determinedairspeed value to change the flight characteristics of the rotorcraft101. The V_(A_RAW) value is used by the control loops for the determinedairspeed.

In some embodiments, the predetermined threshold for the groundspeedincludes hysteresis. For example, the predetermined threshold may be 40knots (or more generally, a first threshold) when the rotorcraft 101 isaccelerating, such that the raw airspeed value (V_(A_RAW)) is used forcontrol until the groundspeed exceed 40 knots, at which time theestimated airspeed (V_(A_EST)) is used for control. Likewise, thepredetermined threshold may be 25 knots (or more generally, a lessersecond threshold) when the rotorcraft 101 is decelerating, such that theestimated airspeed (V_(A_EST)) is for control used until the groundspeedis less than 25 knots, at which time the current raw airspeed value(V_(A_RAW)) is once again used for control. It should be appreciatedthat the thresholds may be varied to optimize response in a givenrotorcraft application.

The method 801 describes a method for performing airspeed estimation bycomplementary filtering when the groundspeed of the rotorcraft 101 isgreater than a predefined threshold. A method similar to the method 801may also be used for performing airspeed estimation by complementaryfiltering when the raw airspeed of the rotorcraft 101 is greater than apredefined threshold. In such embodiments, the raw airspeed may be usedinstead of the groundspeed for the processes 803 through 813. The rawairspeed may be measured by a pitot-static system or other measurementsensor.

FIG. 9 is a block diagram of a filter 901 that may be used to realizethe first system 401 or second system 501, according to someembodiments. The filter 901 is a complimentary filter used to estimatethe airspeed of the rotorcraft 101, and produces an estimated airspeed(V_(A_EST)) signal. The filter 901 is implemented with three integrators903, 905, and 907. Further, the filter 901 is indicative of embodimentswhere one input signal is derived from a transduced sensor signal, andanother signal contains raw airspeed values directly transduced fromanother sensor. For example, a first input signal may be derived from asignal from an accelerometer (e.g., a(t)), and a second input signal maybe a raw airspeed value directly measured by air data systems of therotorcraft 101 (e.g., v(t)), such as with a variometer or other ASI. Thefirst input signal includes raw values for the forward acceleration ofthe rotorcraft 101, and the second input signal includes raw values forthe forward airspeed of the rotorcraft 101. It should be appreciatedthat the filter 901 may be used to estimate either lateral orlongitudinal airspeeds.

Resetting of the integrators 903, 905, and 907 is controlled by aresetter 909. Upon a reset, the integrators 903 and 905 are reset tovalues of zero. The integrator 907 may be reset to one of severalpossible values, as determined by the resetter 909. As noted above,estimation of the rotorcraft 101 airspeed by complementary filtering mayonly be performed when the groundspeed or raw airspeed of the rotorcraft101 is greater than a predefined threshold. When the groundspeed or rawairspeed of the rotorcraft 101 is less than the predefined threshold,the filter 901 is bypassed. The resetter 909 bypasses the filter 901 bycontinually resetting the integrator 907 to the current raw airspeedvalue (e.g., v(t)). When the groundspeed or raw airspeed of therotorcraft 101 is greater than the predefined threshold, the filter 901is permitted to operate normally. Under normal operation, the integrator907 is reset to a delayed value for the estimated airspeed (V_(A_EST)),and the integrators 903, 905, and 907 may be reset under similarconditions as the integrators 703, 705, and 707 (see FIG. 7).

In some embodiments, the resetter 909 includes a mechanism to filter orrate-limit transitions from use of the estimated airspeed value(V_(A_EST)) to use of the raw airspeed value (V_(A_RAW)). Such amechanism may help avoid a step change in the displayed airspeed and mayallow better control of the rotorcraft 101. For example, the resetter909 may include a Constant Period Transient-Free Switch (CPTFS) limiter.In some embodiments, a rate-limiting transition is performed when thefilter 901 transitions from the initialized state (e.g., when the rawairspeed value is used) to the active state (e.g., when the estimatedairspeed value is used), and no rate-limiting transition is performedwhen the filter 901 transitions from the active state to the initializedstate.

Although the embodiments of FIGS. 8 and 9 are described in the contextof estimating the airspeed of the rotorcraft 101, it should beappreciated that embodiment techniques may be used to estimate speedsalong other directions. In particular, the vertical speed could also beestimated using analogous techniques.

Embodiments may achieve advantages. Estimating the vertical speed and/orairspeed of the rotorcraft 101 by combining multiple different signalsmay result in more accurate values than directly measuring raw speedvalues. In particular, by combining multiple noisy signals with acomplementary filtering system, the vertical speed and/or airspeed maybe accurately estimated and flight of the rotorcraft 101 may be bettercontrolled.

Although this invention has been described with reference toillustrative embodiments, this description is not intended to beconstrued in a limiting sense. Various modifications and combinations ofthe illustrative embodiments, as well as other embodiments of theinvention, will be apparent to persons skilled in the art upon referenceto the description. It is therefore intended that the appended claimsencompass any such modifications or embodiments.

What is claimed is:
 1. A method comprising: receiving a first sensorsignal from a pitot tube of a rotorcraft, the first sensor signalincluding raw airspeed values of the rotorcraft; receiving a secondsensor signal from an accelerometer of the rotorcraft; measuring a firstspeed of the rotorcraft, the first speed being groundspeed of therotorcraft or raw airspeed of the rotorcraft; determining airspeedvalues of the rotorcraft, the determining comprising: using the rawairspeed values in the first sensor signal as the determined airspeedvalues in response to the first speed of the rotorcraft being less thanor equal to a predetermined value; and estimating the determinedairspeed values in response to the first speed of the rotorcraft beinggreater than the predetermined value, the estimating comprising:combining the first sensor signal and the second sensor signal with acomplementary filter to determine estimated airspeed values; and usingthe estimated airspeed values as the determined airspeed values; andadjusting flight control devices of the rotorcraft according to thedetermined airspeed values.
 2. The method of claim 1, wherein combiningthe first sensor signal and the second sensor signal comprises:integrating the second sensor signal with respect to time to obtain afirst filtered signal; and adding the first filtered signal and thefirst sensor signal to obtain a second filtered signal, the secondfiltered signal including the estimated airspeed values of therotorcraft.
 3. The method of claim 1, wherein the predetermined value isa first threshold when the rotorcraft is accelerating, and thepredetermined value is a second threshold when the rotorcraft isdecelerating, the second threshold being less than the first threshold.4. The method of claim 3, wherein the first threshold is 40 knots, andthe second threshold is 25 knots.
 5. A rotorcraft comprising: a pitottube; an accelerometer; a plurality of flight control devices; and aflight control computer coupled to the pitot tube, the accelerometer,and the flight control devices, the flight control computer beingconfigured to: receive raw airspeed values of the rotorcraft from thepitot tube; receive raw forward acceleration values of the rotorcraftfrom the accelerometer; measure a first speed of the rotorcraft, thefirst speed being groundspeed of the rotorcraft or raw airspeed of therotorcraft; determine airspeed values of the rotorcraft by: using theraw airspeed values as the determined airspeed values in response to thefirst speed of the rotorcraft being less than or equal to apredetermined value; and estimating the determined airspeed values bycombining the raw airspeed values and the raw forward accelerationvalues in response to the first speed of the rotorcraft being greaterthan the predetermined value; and adjusting flight control devices ofthe rotorcraft according to the determined airspeed values.
 6. Therotorcraft of claim 5, wherein the flight control computer is configuredto estimate the determined airspeed values by: combining the rawairspeed values and the raw forward acceleration values with acomplementary filter to determine estimated airspeed values; and usingthe estimated airspeed values as the determined airspeed values.
 7. Therotorcraft of claim 6, wherein the flight control computer is configuredto combine the raw airspeed values and the raw forward accelerationvalues by: integrating the raw forward acceleration values with respectto time to obtain integrated forward acceleration values; and adding theintegrated forward acceleration values and the raw airspeed values toobtain the estimated airspeed values.
 8. The rotorcraft of claim 5,wherein the predetermined value is a first threshold when the firstspeed of the rotorcraft is less than or equal to the predeterminedvalue, and the predetermined value is a second threshold when the firstspeed of the rotorcraft is greater than the predetermined value, thesecond threshold being less than the first threshold.
 9. A methodcomprising: receiving a first sensor signal from an accelerometer of arotorcraft; receiving a second sensor signal from a pitot tube of therotorcraft; combining the first sensor signal and the second sensorsignal with a complementary filter to determine an estimated speed ofthe rotorcraft; adjusting flight control devices of the rotorcraftaccording to the estimated speed of the rotorcraft; measuring agroundspeed of the rotorcraft; and resetting the complementary filter inresponse to the groundspeed of the rotorcraft being less than apredetermined threshold.
 10. The method of claim 9, wherein theestimated speed of the rotorcraft is an estimate of vertical speed ofthe rotorcraft.
 11. The method of claim 9, wherein the estimated speedof the rotorcraft is an estimate of airspeed of the rotorcraft.
 12. Themethod of claim 9, wherein the combining the first sensor signal and thesecond sensor signal with the complementary filter comprises: filteringthe first sensor signal with a first filter to obtain a first filteredsignal; filtering the second sensor signal with a second filter toobtain a second filtered signal, the second filter and the first filtercomplementing one another; and adding the first filtered signal and thesecond filtered signal to determine the estimated speed of therotorcraft.
 13. The method of claim 9, wherein the combining the firstsensor signal and the second sensor signal with the complementary filtercomprises: subtracting the first sensor signal from the second sensorsignal to obtain a noise difference signal; filtering the noisedifference signal to obtain a filtered difference signal; and adding thefirst sensor signal and the filtered difference signal to determine theestimated speed of the rotorcraft.
 14. The method of claim 9, whereinthe receiving the first sensor signal comprises: receiving anacceleration signal from the accelerometer; and integrating theacceleration signal to obtain the first sensor signal.
 15. The method ofclaim 9, wherein the receiving the second sensor signal comprises:receiving a velocity signal from the pitot tube; and using the velocitysignal as the second sensor signal.
 16. The method of claim 9, whereinthe adjusting the flight control devices of the rotorcraft comprises:decoupling pilot flight controls of the rotorcraft from the flightcontrol devices according to the estimated speed.